1. Field of the Invention
The present invention relates generally to an apparatus for controlling a damping coefficient of a shock absorber (also called a damper, but hereinafter referred to as the shock absorber) interposed between an unsprung mass and sprung mass of the vehicle so as to provide an optimum damping force therefrom. The shock absorber is provided with changing means for changing the damping coefficient thereof at multiple stages in response to a drive signal.
2. Description of the Background Art
A Japanese Patent Application First Publication No. Showa 62-181908 published on Aug. 10, 1987 exemplifies a previously proposed apparatus for controlling a damping coefficient of a vehicular shock absorber.
The previously proposed damping coefficient controlling apparatus recited in the above-identified Japanese Patent Application First Publication includes: acceleration detecting means for detecting a longitudinal acceleration in upward-downward directions of a vehicle body; and command output means for generating and outputting a command to change a damping force to a larger value at a predetermined period of time during which the detected longitudinal acceleration exceeds a predetermined value and the vehicle body returns to a neutral position with respect to the corresponding tire wheel to a damping coefficient changing means installed in the shock absorber. Thus, when a large vibrating force is generated on the vehicle body, the vehicle body can speedily return to its neutral position with respect to the vehicle tire wheel within a half period of the vibration.
Since, however, the previously proposed damping coefficient controlling apparatus has no countermeasure against a failure in its control unit, an abnormal control signal is output from the control unit to an actuator of the damping coefficient changing means and the vehicle exhibits an unnatural behavior if an external noise causes a CPU of the control unit to operate abnormally. Consequently, steering stability of the vehicle and vehicular comfort can be reduced.
In addition, since, in the previously proposed damping coefficient controlling apparatus, a portion for calculating an optimum damping force on the basis for received signals from respective sensors and a portion of generating the control signal to drive the actuator on the basis of the calculation result of the above-described portion are integrated into the control unit, the control signal to drive the actuator cannot be transmitted to the actuator if such a failure as breakage of a wire connected between the control unit and actuator occurs. Even if the control unit detects the abnormality and the actuator is driven at a safe position, the control unit cannot drive the actuator.